Hitherto, in a continuous annealing furnace used for annealing steel strips, for example, when the furnace is started after being opened to air or when air enters the furnace atmosphere, in order to reduce the moisture and oxygen concentration in the furnace, a method in which non-oxidizing gas such as inert gas, which serves as gas replacing the furnace atmosphere, is supplied into the furnace and simultaneously the gas in the furnace is exhausted in order to replace the furnace atmosphere by the non-oxidizing gas while the furnace temperature is increased in order to vaporize the moisture in the furnace has been widely employed.
However, in the case where the existing method described above is employed, there is a problem in that productivity may be reduced considerably because a long period of time is required to reduce the moisture and oxygen concentration in the furnace atmosphere to a certain level that is suitable for the normal operation of the furnace and it is impossible to operate the furnace during that period of time.
On the other hand, recently, there has been an increase in demands for high-tensile steel (high-tensile material) that contributes to the fields of automobiles, home appliances, building materials, and the like by, for example, reducing the weight of a structure. In the technique for manufacturing high-tensile materials, it is indicated that there is a possibility that high-tensile steel strips having good hole expandability can be manufactured by adding Si to steel. In the technique for manufacturing high-tensile materials, it is also indicated that there is a possibility that steel strips in which retained γ is likely to be formed and which has high ductility can be provided by adding Si or Al to steel.
However, in the case where a high-strength cold-rolled steel strip contains an oxidizable element such as Si or Mn, there is a problem in that the oxidizable element concentrates at the surface of the steel strip during annealing and thereby forms an oxide of Si, Mn, or the like, which may disadvantageously result in poor appearance and deteriorate ease of a chemical conversion treatment such as a phosphate treatment.
In manufacture of hot-dip galvanizing steel strips, in the case where the steel strip contains an oxidizable element such as Si, Mn, or the like, there is a problem in that the oxidizable element concentrates at the surface of the steel strip during annealing and thereby forms an oxide of Si, Mn, or the like. This may deteriorate ease of plating, which causes plating defects. In addition, when an alloying treatment is performed after plating, the alloying rate may be reduced. In particular, Si considerably reduces the wettability of the steel strip with a molten plating metal when Si forms an oxide film of SiO2 on the surface of the steel strip. Furthermore, the oxide film of SiO2 acts as a barrier to diffusion of the plated metal and the base steel during the alloying treatment. Thus, Si is especially likely to deteriorate ease of plating and degrade ease of an alloying treatment.
As a method for avoiding the above-described problems, a method in which the oxygen potential in the annealing atmosphere is controlled is considered.
As a method in which the oxygen potential is increased, for example, Patent Literature 1 discloses a method in which the dew point in a region from the latter part of a heating zone to a soaking zone is controlled to a high dew point of −30° C. or more. This method works to some extent and is advantageous in that controlling the dew point to be high can be achieved industrially easily. However, this method is disadvantageous in that a certain type of steel (e.g., Ti—IF steel) that is undesirably subjected to an operation under a high dew point is not able to be manufactured easily by this method. This is because lowering the dew point of an annealing atmosphere which has been increased to a high dew point once to a low dew point requires a quite long period of time. Moreover, since the furnace atmosphere is set oxidative in this method, there is a problem in that a mistake in controlling the dew point may cause an oxide to adhere to the rolls disposed in the furnace, which causes pick-up defects, and there is also a problem of damages to the furnace wall.
As another method, a method in which the oxygen potential is reduced may be proposed. However, since Si, Mn, and the like are quite oxidative, it has been considered that it is very difficult to consistently achieve atmosphere having a low dew point of −40° C. or less which markedly suppresses oxidation of Si, Mn, or the like in a large continuous-annealing furnace installed in a CGL (continuous hot-dip galvanizing line) or a CAL (continuous annealing line).
Techniques for preparing annealing atmosphere having a low dew point with efficiency are disclosed in, for example, Patent Literatures 2 and 3. These techniques are directed to one-pass vertical-type furnaces, that is, relatively small furnaces but are not supposed to be applied to multipass vertical-type furnaces such as a CGL and a CAL. Therefore, there is a high risk that the dew point fails to be efficiently lowered by the technique.